1. Field of Invention
This invention relates to window glass equipped with thermal defogging wires for use in motor vehicles and in particular to means for suppressing the noise generated in the thermal defogging wires.
2. Description of the Prior Art
In recent years motor vehicles have been equipped with various electronic devices such as FM radios, radio telephones, electronically controlled fuel injection systems, etc. Since such devices are operated by signals in the high frequency range, they have suffered from a drawback of being susceptible to the detrimental effects of high frequency noise currents. Such noise currents are generated by the ignition system, the voltage regulator, and various other switches such as the horn relay, turn signals, etc. Since the electronic devices are susceptible to the detrimental effects of high frequency noise currents, counter measures are needed to reduce or eliminate the detrimental effects.
Particularly, noise currents are enhanced in the case of a motor vehicle window glass which has thermal defogging wires either embedded in it or attached to it. It is well known that these thermal defogging wires become secondary radiators which radiate noise currents that have built up in the defogger current supply wire. Furthermore, with the advent of radio antennas that are also contained in the window glass, the susceptibility to the harmful effects of noise generated by the thermal defogging wires is greatly increased. The installation of both the radio antenna and the thermal defogging wires in window glass is a very desirable feature, but the amount of noise generated by the thermal defogging wires is undesirably large. This undesirably large noise current in the defogging wires causes a noticeable drop in the signal to noise ratio of the signals received by the antenna and has made the use of such antennas almost impractical.
In the prior art, noise prevention devices, such as condensers, that bypass the high frequency noise current, have been attached to the source of high frequency noise currents for the purpose of noise suppression. Other means of noise suppression include separating the current supply wires for the thermal defogging wires and the wires in which noise currents are built up, or to avoid the coupling of noise currents from one wire to another by means of shielded wires, etc. However, these prior art methods have not only been insufficiently effective but have also suffered from the drawback of requiring the installation of special devices.
This invention is the result of lessons learned in connection with the above mentioned difficulties that have been encountered in the prior art.